1. Field of the Invention
This invention relates to a process for producing an Mg-based composite material or an Mg alloy-based composite material.
2. Description of the Related Art
Conventionally, various techniques such as melt stirring, powder metallurgy and melt forging (or squeeze casting) have been widely used to produce Mg-based composite materials and Mg alloy-based composite materials. Among others, melt forging is mainly employed for the production of composite materials using a light metal as the matrix, because of its relatively good mass productivity. However, since melt forging involves the pressurization of a melt under a high pressure, large-scale and expensive equipment is required which causes an increase in the production cost of the composite materials. This constitutes a serious obstacle to the practical use of such composite materials.
FIG. 7 is a sectional view illustrating a conventional process for producing an Mg-based composite material or an Mg alloy-based composite material according to the melt forging technique. When a melt is handled as in this melt forging technique, a measure is usually taken to prevent the combustion of the melt by using SF.sub.6 gas. Now, the aforesaid process for producing a composite material is described with reference to FIG. 7. First of all, a melt 103 of Mg or Mg alloy is poured into a metal mold 102 having a mass of reinforcing material 101 placed therein. Then, melt 103 is pressurized with a plunger 104 to form a composite material. In order to prevent the oxidation and combustion of melt 103, a protective gas (e.g., a gaseous mixture composed of CO.sub.2 and 0.5% by volume of SF.sub.6) 105 is previously introduced into metal mold 102.
When attention is paid to the interface between the reinforcing material and the melt of Mg or Mg alloy during the formation of a composite material, it can be seen that, in the conventional process, the reinforcing material first comes into contact with a stable (inactive) protective film present on the surface of the melt. Although this protective film is considered to be a thin film of a complex compound consisting of Mg (magnesium), O (oxygen), F (fluorine) and S (sulfur), the reinforcing material has poor wettability by the protective film and, therefore, requires the above-described pressurization of the melt during the formation of a composite material. Moreover, even if the protective film is once destroyed as a result of the pressurization of the melt, the protective gas present within the mass of reinforcing material causes a fresh protective film to be instantaneously formed on the surface of the melt. Consequently, it is necessary to pressurize the melt continuously.
In order to solve the above-described problems, it is necessary to improve the wettability of the reinforcing material by the melt of matrix metal. Such an improvement would make it possible to reduce the pressure applied to the melt or omit the pressurization of the melt. Accordingly, considerable efforts have hitherto been exerted to improve the wettability of the reinforcing material by the melt of matrix metal. However, a process which simultaneously satisfies the requirements including mass productivity, low cost and the soundness of the composite material has not been established as yet.
As proposed, for example, in Japanese Patent Publication No. 7-100834/'95, Japanese Patent No. 2576186, Japanese Patent No. 2576188, and Japanese Patent Provisional Publication No. 1-279721/'89, there is known a process in which a shaped body comprising a reinforcing material having a metal oxide, finely divided metal or metal fluoride mixed uniformly therewith is formed, and at least a part of the shaped body is brought into contact with a melt of Mg or Mg alloy so as to infiltrate the melt into the shaped body without pressurization. This process has merits from a manufacturing point of view in that the necessity of the equipment for pressurizing the melt is eliminated, the metal mold for casting use becomes unnecessary under certain conditions, and the casting yield is enhanced. Accordingly, this process is improved from the viewpoint of mass productivity and cost reduction.
However, in many cases, the metal oxide, finely divided metal or metal fluoride itself and the reaction product of this substance with the melt of matrix metal constitute impurities in the resulting composite material, resulting in a reduction in the characteristics thereof. Accordingly, the above-described process is unsatisfactory from the viewpoint of the soundness of the composite material. Moreover, it also has the disadvantage that the metal oxide, finely divided metal or metal fluoride itself causes an increase in cost.